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What Does DSLR Stand For?
DSLR stands for ddigital single-lens reflex. Sound complicated? Well, DSLRs use a series of mirrors to reflect light through a single prism, which then refracts the light into the light sensors. That's it!
Speaking old school photography here, one would typically use the viewfinder to, well, view the image. In DSLRs, the light comes through the camera via a series of lenses, including a “4 elements lens" that includes two convex and two concave lenses. This is then reflected by the “reflex mirror"--where DSLRs get their name—across a series of mirrors into the viewfinder and the eye of the photographer.
To take a picture, the aperture size either increases or decreases according to the desired setting and the same reflex mirror slides out of the way of the light as the shutter opens. The image sensors then take in light for the duration of the exposure. This explains the “viewfinder blackout" that one gets as one takes the picture while looking through the viewfinder as the mirror flips up and no longer reflects light into your eye. Most of the time lapse in taking a photo is getting all those mirrors and apertures to move more quickly, because even milliseconds is too much of a drag for many action photographers.
Of course, there are just plain old SLRs without the digital "D" - these are film cameras. It is for these that viewfinders were first designed as we know them in DSLRs, because alternatives like LCD screens didn't exist until quite recently. However, these are not the focus of the article.
There are other features that are associated with DSLRs, even though they aren't a part of the technical definition:
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Another difference with most DSLR cameras is that they tend to have larger, better sensors than other types of digital cameras, which most professional photographers prefer. This is mostly where the (generally) superior quality of DSLRs come from: not so much as the length of the barrel as the quality of the sensors.
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Another common feature of DSLRs is the ability to mount different lenses on the end, from whopper telephotos to entirely featureless clear filters. Point & shoots simply don't have this capability without some careful hacks.
DSLRs also just come with better lenses to start with, hence their heftier size. This allows for more precise control of depth of field.
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DSLR: The Difference
So, what's the big difference between a DSLR and a point and shoot, just going by the technical definition discussed earlier? Well, point & shoots don't tend to have viewfinders and mirror arrays in the same sense as DSLRs, in that the LCD screen displays whatever the image sensors are detecting, which are continuously exposed to light. Adjustments are made to the preview to estimate what the image would actually look like in terms of aperture and other settings, which is what you see on the LCD screen.
Many DSLRs are feature similar set ups, often with no viewfinder available at all. While this bothers many people who prefer viewfinders, others prefer the ability to use LCDs. This seems to becoming default since the introduction of the first such one, the Olympus E-10, back in 2000.
While this technically disqualifies them from the definition of a DSLR—no reflex mirror, no single lens reflex—people seem to keep on calling this style of camera DSLR. The meaning of the name itself is not as important compared to the style of camera attached to it.